1. Field of the Invention
The present invention relates to a carbon electrode for a nonaqueous secondary battery, a fabrication method for the same and a nonaqueous secondary battery using the same. More particularly, the present invention relates to a carbon electrode for a nonaqueous secondary battery which has a high capacity and long life and is safe, a fabrication method for the same and a nonaqueous secondary battery using the same. The nonaqueous secondary battery of the present invention can be suitably used for driving electric sources for portable equipment such as electronic equipment, information equipment and the like.
2. Related Art
As miniaturization and weight reduction of portable equipment such as electronic equipment and information equipment rapidly progress, the secondary batteries which drive them have become increasingly important.
Because a lithium secondary battery is lightweight and has high energy density, it is considered promising as a driving electric source for portable equipment, therefore research and development thereof have been actively progressing. However, when a simple substance of lithium metal is used for a negative electrode, repetition of a charge and discharge cycle causes dentrites to generate and grow on the lithium metal, which lead to an internal short circuit, for which reason it is difficult to use lithium metal for a secondary battery. Also, lithium alloys such as lithium-aluminum alloy are proposed in place of lithium metal. However, because charge and discharge cycles or deep charging/discharging causes segregation of the alloy, sufficiently satisfactory characteristics are not obtained.
Accordingly, a battery using a negative electrode which utilizes the intercalation-deintercalation reaction of lithium ion with carbon as a host material was proposed. The research and development thereof has been progressing and has to a certain extent been put to practical use. A lithium secondary battery in which carbon is used for a negative electrode is excellent in terms of its cycle characteristic and safety. However, not only carbon materials have large variations in their structure carbon, but the physical properties and texture thereof greatly influence the performance of the electrode, which allows various types of carbon electrodes to be proposed.
For example, electrodes using relatively amorphous carbon for a negative electrode as shown in JP-A-61-111907 (the term "JP-A" as used herein means an unexamined published Japanese patent application) and JP-A-62-90863, electrodes using graphite for a negative electrode as shown in JP-A-60-182670, JP-A-60-221964, JP-A-4-155776 and JP-A-4-115467, and electrodes in which attention is paid to the texture of carbon, not the crystallinity of carbon as shown in JP-A-4-280068 and JP-A-4-342958 have been proposed. However, because all of these carbons are powdery or fibrous, a binder has to be mixed when they are used in electrodes. Accordingly, even if the carbon itself has excellent performance, problems will remain in terms of the cycle characteristic and amount of active material per volume when it is put to practical use in an electrode.
Meanwhile, a method in which carbon is deposited directly on a material capable of serving as a collector by a chemical vapor deposition method is proposed in JP-A-60-36315 and U.S. Pat. No. 4,863,814. The carbon electrodes produced by these methods show excellent characteristics. In particular, those prepared by depositing carbon directly on a metallic substrate not only do not require a binder but also have good current collection and therefore have high capacity and exhibit high cycle stability.
Further, those prepared by carrying a substance having a catalytic action with respect to polymerization for monomer on current collector as shown in JP-A-59-18578, or those prepared by depositing carbon on a substrate having a catalytic action on graphitization of carbon materials as shown in U.S. Pat. No. 4,968,527 and U.S. Pat. No. 4,863,818 are available. In the former, polymer are used as a battery-active material, and the catalytic action is for polymerization of monomer. In the latter, however, it is known that because carbon having high graphitization is deposited at low temperatures with strong adhesion, carbon electrodes having high capacity and an excellent cycle characteristic are obtained.
It is described in JP-A-4-92364 and JP-A-5-347155 that organic substances are impregnated into metallic porous bodies and carbonized to obtain carbon electrodes.
However, when carbon is formed either from a gas phase or a liquid phase, there is the defect that, when carbon is formed directly on the substrate, it is formed over the entire substrate. That is, in the case where electrodes prepared by depositing carbon directly on a conductive substrate having a catalytic action on the graphitization of carbon materials are used to produce batteries, a process for peeling off carbon deposited on a welded portion is required when welding the battery case and electrode. Also, applying a mask in order to make a portion where carbon is not deposited has been considered, but this masking is actually very difficult at the high temperatures required to form carbon electrodes by depositing carbon from a carbon source. Further, when the conductive substrate having a catalytic action is used, it is difficult to peel the carbon off because the carbon is deposited with strong adhesion.
As the need for a higher capacity battery increases, reducing the thickness of a metallic plate which is a collector (conductive substrate) or use of a three-dimensional structure in order to obtain a large current is required. However, in the electrode fabrication methods described above, because the collector metal itself acts as a catalyst, catalytic atom are caught in carbon when forming the carbon electrode, and therefore the collector becomes thin or narrow. Such a phenomenon, while giving sufficient current collection to an active material, makes it difficult to peel off only carbon deposited at a weld portion when carrying out welding to the battery case. Also, welding as is, without peeling the carbon, has caused problems in terms of contact resistance and strength, which in turn leads to the defects that the internal resistance of the battery increases and fabrication yield is lowered and, further, that reliability is greatly influenced.